Data from: Small N e of the isolated and unmanaged horse population on Sable Island

For small, isolated populations 2 common conservation concerns relate to genetic threats: inbreeding and negative consequences associated with loss of genetic diversity due to drift. Mitigating these threats often involves conservation actions that can be controversial, such as translocations or captive breeding programs. Although such actions have been successful in some situations, in others they have had undesirable outcomes. Here, we estimated the effective population size (N e) of the Sable Island horses to assess the risk to this population of these genetic threats. We found surprising consistency of N e estimates across the 5 different methods used, with a mean of 48 effective individuals. This estimate falls below the 50 criterion of the “50/500 rule,” below which inbreeding depression is a concern for population viability. However, simulations and knowledge of population history indicate that this population is still in its early stages of approaching equilibrium between mutation, drift, and genetic diversity; and no negative consequences have been identified that could be associated with inbreeding depression. Therefore, we do not recommend taking management action (such as translocations) at this stage. Rather, we propose continued monitoring of genetic diversity and fitness over time so that trends and any substantial changes can be detected. This represents one of the few unmanaged horse populations in the world, and therefore these data will not only alert us to serious concerns regarding their conservation status, but will also provide a wealth of information about how natural processes drive patterns of reproduction, mortality, and population growth over time.

针对小型孤立种群，两类常见的保护遗传学威胁尤为突出：一是近交问题，二是遗传漂变引发的遗传多样性丧失所带来的负面效应。缓解此类威胁的保护干预措施往往颇具争议，例如种群易地转移（translocations）与人工繁育计划（captive breeding programs）。尽管这类手段在部分场景中已取得成功，但在其他情境下却会产生非预期的不良后果。本研究对萨布尔岛野马（Sable Island horses）的有效种群大小（effective population size，记作Nₑ）进行了估算，以评估该种群面临上述遗传威胁的风险。研究发现，采用5种不同方法得到的Nₑ估算结果具有惊人的一致性，平均有效种群个体数为48。该数值低于"50/500法则"（50/500 rule）所设定的临界值，当种群有效大小低于该阈值时，近交衰退（inbreeding depression）便会对种群生存力（population viability）构成威胁。不过，种群模拟结果与种群历史研究均表明，该种群仍处于突变、遗传漂变与遗传多样性之间达到平衡的早期阶段；截至目前，尚未发现与近交衰退相关的负面效应。因此，我们暂不建议在此阶段采取诸如种群易地转移之类的管理干预措施。相反，我们提议持续开展长期监测，以跟踪种群的遗传多样性与适合度变化，从而及时发现种群趋势变动及任何重大异常。萨布尔岛野马种群是全球少数未受人工管理的马种群之一，因此本研究获取的数据不仅能够为我们预警该种群保护现状中的严峻问题，还将为阐明自然过程如何随时间塑造繁殖、死亡与种群增长模式提供丰富的研究资料。